Delayed warm-up filament power supply for mass spectrometer with emission current regulation



y 1965 R. WATTERS 3,197,634

DELAYED WARM-UP FILAMENT POWER SUPPLY FOR MASS SPECTROMETER WITHEMISSION CURRENT REGULATION Filed Oct. 30, 1963 [/3 [Q ElectronCollector 3 H Electric Power Z -l flegu/afor and Ourpul MassSpectrograph 2 Pane! Source 4 9 Filament Auxiliary J: 8 l6 6 7PowerSaurce T /5 30 7'0 Filament I TOE/ec/r0n I Collector I c 0 l 50 I5/ I 3 J 3 F 'g a I S lnvenfor Robe/l L. Walters N Time, in. HisAttorney.

United States Patent DELAYED WARP/LU? FEAMENT PGWER SUPPLY The presentinvention relates to protective means for thermionic electron-emissivefilaments, and more particularly pertains to means for increasing theuseable lifetime of such filaments.

The mass spectrometer has proved to be a valuable analytical tool inmany fields of scientific endeavor. In the mass spectrometer a smallthermionic electron-emissive filament is used to provide a source ofelectrons which ionize particles in the electron stream. The ionized, orcharged, particles are thereafter accelerated under the infiuence of anelectricfield into a magnetic field where the particles follow a curvedtrajectory. The curvature of the trajectory depends primarily upon thevelocity of the particle, the magnetic field strength, and the means ofthe particle. For a given field strength and velocity, heavier particlescurve less than lighter particles. Hence, the particles are separatedaccording to their masses. Suitable indicating means are used to measurethe relative number of each weight of particle present, and this isrelated to the atomic structure of the particles. Thus, the massspectrometer is used to analyze the elements and compounds present inthe vicinity of the filament.

In the past, numerous difficulties have been encoun tered in massspectrometers because of early filament failure. For this reason thedevices are frequently supplied with dual filaments which may be usedsuccessively. Still, the total filament life is short, the cost ofreplacement high and the time required for replacement long. Forexample, the normal time required for filament replacement is in excessof three weeks, which is a relatively long time to have expensiveequipment inoperative. There is a definite need for means to providelonger life for the filaments in mass spectrometers.

Accordingly, it is a primary object of this invention to provide meansfor increasing the lifetime of filaments in mass spectrometers.

it is another object of this invention to provide increased lifetime forthe filament in mass spectrometers by means independent of operatorerror.

Briefly, I discovered that the reduced lifetime of filaments in massspectrometers is due in large part to the high magnitude of filamentcurrent demanded by the emission current regulator when the massspectrometer is initially energized. Accordingly, with the presentinvention, I provide an emission current regulated power supply for thefilament which delays the application of full power to the filament. fora predetermined time during which a gradual increase in power occurs.The time delay is accomplished, preferably, with a source of graduallyincreasing voltage which is used to supplement the electron collectorinput signal to the control element of the filament power supply. Aftera predetermined time lapse, the auxiliary voltage stabilizes at aconstant value, in order to minimize any disturbance of the emissioncurrent control function, and more particularly to avoid adverselyaffecting the rapid response time thereof.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof, may best be under- 3,197,634Patented July 27, 1965 stood by reference to the following descriptiontaken in connection with the accompanying drawings in which:

FIGURE 1 is a block diagram which shows the system organization of theemission current regulated power supply of my invention;

FIGURE 2 is a schematic circuit diagram of a specific emission currentregulated power supply, which I prefer to use, in accordance with thepresent invention; and,

FIGURE 3 is a graph of filament current versus time using the powersupply of FIGURE 2.

It is known that the thermionic, electron emissive filaments of massspectrographs normally have a relatively short useful life. It is alsoknown that the expense of replacing the filament in mass spectrographtubes is approximately /5 the cost of a new tube, for the smallermodels, and that ordinarily such replacement may not be effected in thelaboratory where such equipments are used, necessitating shipment to arepair facility. In most cases, the cost of mass spectrograph devices isso high that a standby, or substitute, cannot be justified. Accordingly,premature filament failure results in high replacement expense and along time delay during which the equipment is inoperative.

it is known that the useful lifetime of relatively long, thin electronemissive filaments oftentimes can be extended by providing a slowwarm-up or reduced voltage operation during initial energization. Thereason for this is that the mechanical shock of the initial inrushingcurrent is frequently the cause of a mechanical failure or fracture, ofthe filament. The initial current surge in tungsten, for example, issometimes as high as 15 times the normal current at the high operatingtemperature. A common example of such mechanical failure is to be foundwith the incandescent filament of the ordinary light bulb. Such lightbulbs fail, almost without exception, during initial energizationthereof and very rarely expire during sustained operation.

Although it was known that current surge could create mechanicalproblems in filaments having a relatively long length relative to theircross sections, it was not considered, prior to my discovery thereof,that initial current inrush caused the reduced useful life in thefilaments of mass spectrometers. One reason for this is that ion gauges,which utilize similar emission current regulated filament supplies,provide long useful lives with relatively long thin filaments. Thefilaments of mass spec trometers. are relatively short and have thickcross sections which would be thought normally to minimize breakage bymechanical stress due to current surge.

In the light of my discovery that reduced filament power during thewarm-up period increases mass spectrometer filament lifetime, it appearsthat filament failure previously was caused by a lack of thermalequilibrium throughout the filament during the warm-up period. Thus, therelatively large electrically conductive means, which are normallyconnected to the ends of the low voltage, high current filament,provided a high capacity eat sink which maintained the ends of thefilament much cooler than the center thereof. Thus, when the emissioncurrent regulator sensed the reduced emission, which normally occursduring warm-up, full filament power was applied to the filament and theenergy thereof dissipated primarily in the central portion of thefilament. In this way, the central portion of the filament was reducedin cross section by loss of material therefrom as a consequence of thehigh energy dissipated therein. Thereafter, the central portion of thefilament exhibited a higher resistance than the end portions, resultingin a further unbalance of power dissipation even after warmup had beencompleted. Thus, even when a relatively small reduction in cross sectionoccurred during warm up, the deleterious effect was compounded duringnormal operation.

It should be understood that'the foregoing explanation isofiiered as themost likely cause of prior filament failures and the present inventionis not dependent upon the correctness thereof. The significant fact ismy discovery that gradually increased power during warm-up markedlyincreases the useful lifetime of the high current, low voltage filamentsused in mass spectrometers.

FIGURE 1 illustrates, by a block diagram, an emission current regulatedpower supply in accordance with the present invention. The power supplyincludes an electric power regulator 1 which has a source of electricpower and control means. Control terminal 2 provides means for supplyinga power input to regulator 1. Preferably, regulator 1 is responsive tovariations in the magnitude of electric power supplied to controlterminal 2 to provide a variation in the same direction in the magnitudeof electric power available at the regulator output means, convenientlyshown as terminals 3 and Regulator 1 may be any of a plurality ofcurrent controlling means including a magnetic amplifier, vacuum tube ortransistor power amplifier, and a thyratron or silicon controlledrectifier power controlling circuit. I prefer to use a transistor seriesregulator using a power transistor because such devices are particularlywell suited for low voltage, high current regulation as required for usewith mass spectrometer filaments.

Electrically conductive means, which may conveniently be provided byhigh current capacity braided copper straps, are shown schematically asconductors 5 and 6. Conductors 5 and 6 electrically connect regulatoroutput terminals 3 and 4 to the terminals '7 and 8, respectively, ofthermionic, electron-emissive filament 9. Filament 9, of massspectrograph 1Q, normally has a length which is in the same order ofmagnitude as its cross-sectional di mension. This configuration isrequired to provide a concentrated source of electrons and carries withit the additional requirement that the "filament be a low voltage highcurrent device. Filament 9 provides a quantity of electrons bythermionic emission which varies in mag-- nitude in the same directionas variations in the magnitude of power delivered from output terminals3 and 4, as is well-known in the art.

Spectrograph 10 includes an electron collector 11 which receivessubstantially all of the electrons emitted by filament 9. Collector 11is electrically connected to collector terminal 12 externally of theenvelope of spectrograph 10. Conductive means 13,'which may be, forexample, a copper wire, electrically connect collector electrode 12 tocontrol terminal 2 of regulator 1.

In accordance with the present invention an auxiliary source of electricpower 14 is provided having output 15 thereof connected, by conductivemeans 15 to control terminal 2 of regulator 1. Source 14 provides apower 'output at terminal 15 which gradually increases between the timeof energization thereof and a predetermined time at which full outputpower capability is reached. The output power increases gradually duringthe delay period, providing an output not unlike that provided by whatis known in the art as a ramp generator, owing to the shape of thevoltage waveform. At the end of the delay period, when the predeterminedtime has expired, the power output from source 14 is essentiallyconstant.

Operation of the system of FIGURE .1 is as follows. When the system isinitially energized, there is no power output from source 14 to controlterminal 2. Also, no electrons are being emitted from filament 9 to becollected by electron collector 11 and, consequently, no power flowsfrom terminal 12 to control terminal 2. Thus,

regulator. 1 is supplying no power to filament 9. There after, anincreasing magnitude of power is supplied by source 14 to controlterminal 2. In response to the input power to control terminal 2,regulator ll generates an output between terminals 3 and 4 whichenergizes filament 9. Thus, the filament power is applied graduallyuntil a predetermined time after which the output from source 14 isessentially constant. Control is then relinquished to electron collector11. The power flowing in the circuit including collector 11 is arrangedto have a much lesser influence on regulator 1 than the power suppliedby source 14, and the power in the electron collector circuit serves toregulate the emission from filament 9.

The power in the electron collector circuit is arranged to provide avariation in the opposite direction from the power supplied by source1.4. This may be accomplished, conveniently, by providing a source 14having a positive polarity output since, then, an increased quantity ofelectrons to collector 11 results in a less positive potential appliedto control terminal 2. Thus, as the emission from filament 9 increases,the control power to terminal 2, and regulator 1, decreases, reducingthe power to and emission from filament 9. In this way, the emissioncurrent is regulated, or maintained substantially constant. The powersupplied to control terminal 2 by electron collector 11 may beconsidered as providing a small modulation of the power supplied tocontrol terminal 2 by source 14.

FIGURE 2 is a schematic circuit diagram of an emission current regulatedpower supply which I prefer to use in the practice of my invention. Theelectric power regulator 1 is a series transistor regulator. The powersource for regulator 1 includes a transformer 20, full- :ave rectifyingbridge 21 electrically connected thereto, and current filtering meansprovided by resistor 22 and capacitor 23.

Regulation is provided by control of power transistors 25 which have,preferably, load balancing resistors such as resistors 26 and 27connected in series in their emitter circuit paths. Of course, onetransistor will suffice in many lower power applications and others,requiring higher power, may advantageously include more than two.

Transistor 28, which is electrically connected in series in the basecircuits of transistors 24 and 25 controls the impedance between theprincipal electrodes (collectors and emitters) of the power transistors.The circuit connection is such that as the base electrode 29 oftransistor 28 is caused to assume a more positive potential, transistor28 and transistors 24 and 25 conduct more heavily providing an increasedpower output at regulator output terminals 30 and 31.

Auxiliary power source 14 includes a tranfsormer 32 connected tofull-wave bridge circuit 33 to provide direct current conversion.Capacitor 36 and resistor 37 serve as a filter. Resistor 38 provides adischarge path for the delay means to be presently described. Inaccordance with the present invention the power supplied to outputterminals 34 and 35 of source 14 is caused to increase gradually.

Delay in power supplied by output terminals 34 and 35 is secured by adelay network connected to the base electrode 39 of transistor 40, whichhas its principal electrodes connected in series with the output fromterminal 34. The delay is achieved by the relatively long time requiredfor large capacitors 41 and 42 to charge through resistors 43 and 44,which have a relatively high value of resistance. A resistor 45 connectsone principal electrode of transistor 40 to output terminal 34. A zenerdiode 46 and a capacitor 47 are connected in parallel and shuntingoutput terminals 34 and 35. Zener diode 46 provides a substantiallyconstant voltage output from terminals A and B after the initial delayhas occurred and capacitor 47 reduces the high frequency noise voltagenormally generated in zener diodes. In order to effect initial controlof the regulator by auxiliary source 14, positive terminal 35 of source14 is connected to base 29 of transistor 28 and terminal 34 is connectedto serially disposed resistors 50 and 51, which provide a return path tothe emitter of transistor 28. In addition, terminal 34 is connectedthrough a direct current power supply, generally designated at 52, tothe electron collector of the mass spectrograph. Thus, the electroncollector output signal is in series with the auxiliary source in thecircuit of FIGURE 2, whereas it was in parallel therewith in the diagramof FIGURE 1. Power supply 52 could be replaced, conveniently, by abattery. Supply 52 serves to maintain the electron collector at asufficiently high potential to effect collection of electrons thereon.

Operation of the circuit of FIGURE 2 is similar to that previouslydescribed in connection with FIGURE 1 and, for the sake of brevity, willbe only briefly treated herein. When the system is energized byconnecting a source of electrical energy to transformer input terminals53 and 54, initially negligible current flows to the filament of themass spectrometer through terminals 30 and 31. This is because no poweris initially supplied from terminals 34 and 35 of source 14 or from theelectron collector to base 29 of transistor 28. Therefore, base 29 oftransistor 28 continues to assume a potential equal to that of itsemitter and no current flows between the principal electrodes oftransistor 28. Accordingly, no current flows in the base circuits ofpower transistors 24 and 25 and the resistance between their respectiveprincipal electrodes is high enough to permit only a negligible flow ofcurrent through terminals and 31.

Thereafter, the slowly rising positive potential supplied to base 29 oftransistor 23 causes increasing conduction in this transistor and inpower transistors 24 and 25, resulting in a gradually increasing powersupplied to the filament through terminals 30 and 31. At a predeterminedtime, primazily determined by the value of the resistors and capacitorsin the time delay networks of source 14, the voltage between terminals34 and 35 ceases to increase and assumes a constant magnitude determinedby zener diode 4-6. Thus, the filament power is gradually increased overa prolonged time, efiecting protection of and longer life for thefilament of the mass spectrometer.

After the predetermined time has elapsed, terminals 34 and 34 provide acircuit element substantially identical to a constant voltage, lowimpedance battery. Thus, power in the electron collector circuit isreadily transmitted to base 29 of transistor 28 through terminals 34 and35. It is of particular significance to note that the low alternatingcurrent impedance between terminals 34 and 35 ensures that control offilament power in response to rapid variations in electron collectorpower can be effected. In other words, the time response of the emissioncurrent regulated power supply is not deleteriously affected after thepredetermined warm-up time has been consumed.

A particularly desirable emission current regulated power supply inaccordance with my invention as shown in FIGURE 2 utilized the followingspecific components, which are given merely to aid those skilled in theart in the practice of my invention:

Regulator 1 Transformer 2% Secondary Winding, 8 volts R.M.S. 6 amperes.Rectifying bridge 21-4 1N1349A diodes.

Transistors 2d and 25 Type 2Nl550. Transistor 28 Type 2N498. Resistor 220.3 ohm.

Resistor 26 0.3 ohm.

Resistor 2,7 0.3 ohm.

Resistor 50 10,000 ohms. Resistor 51 3,000ohms. Capacitor 23 4,000microf-arads.

6. Auxiliary source 14 Transformer 32 Secondary winding, 12% voltsR.M.S. milliamperes. Rectifying bridge 33-4 1Nl696 diodes.

Transistor 40 Type 2N43. Resistor 37 100 ohms. Resistor 38 1,000 ohms.Resistor 43 15,000 ohms. Resistor 4d 15,000 ohms. Resistor 45 ohms.Capacitor 36 2,000 microfarads. Capacitor 41 500 microfarads. Capacitor42 500 microfarads. Capacitor 47 20 microfarads. Zener diode 46 1Nl523.

Power supply 52 70 volts l milliampere DC. output between terminals Cand D.

FIGURE 3 is a graph showing filament current in amperes versus time inseconds using the specific embodiment detailed above to supply a massspectrometer filament rated at two volts and siX amperes. While massspectrometer filaments of this type were previously attaining usefullifetimes of approximately one hour with six stops and starts, thefilament energized in accordance with the present invention is yet inuse after 50 hours of operation which included 30 tops and starts.

While only certain preferred features of the invention have been shownby way of specific illustration, many modifications and changes willoccur to those skilled in the art. It is, therefore, to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An emission current regulated power supply for the filament of a massspectrometer having a thermionic electron emissive filament and anelectron collecting electrode, said power supply comprising:

(a) an electric power regulator including a source of electric power andhaving electric power output means and control means, said regulatorbeing responsive to variations in the magnitude of electric powersupplied to said control means to provide a variation in the samedirection in the magnitude of electric power delivered by said outputmeans;

(b) electrically conductive means connecting said filament to saidelectric power output means for energization thereby, said filamentproviding a quantity of electrons which varies in magnitude in the samedirection as variations in the magnitude of power from said outputmeans;

(c) an auxiliary source of electric power having output means, saidauxiliary source providing a delay between the time of energizationthereof and a predetermined time at which full output power capabilityis reached, said output power capability increasing gradually to fulloutput power capability and thereafter being essentially constant; and

(d) conductive means connecting said electron collecting electrode andthe output means of said auxiliary source to said control means, saidauxiliary source being arranged to provide an electric power input tosaid control means which efii'ects a gradual increase in the powersupplied to said filament until said predetermined time elapses andthereafter provide a constant magnitude power input to said controlmeans, said electron collecting electrode being arranged to providevariations in the magnitude of power to said control means which are inthe opposite direction to variations in the quantity of electronsemitted by said filament and received by said ca era (a) a seriestransistor regulator connected to a source of unidirectional c rrent andhaving electric current output means and control means, said regulatorbeing responsive to variations in the magnitude of electric voltagesupplied to said control means to provide a variation in the samedirection in the magnitude of electric current delivered by said outputmeans; (1)) electrically conductive means connecting said fila- 7electron collecting electrode to effect substantially constant emissioncurrent from said filament after said predetermined time has elapsed.

2. An emission current regulated power supply'for the filament of a massspectrometer having a thermionic electron emissive filament and anelectron collecting electrode, said power supply comprising:

(a) an electric power regulator connected to a source of electric powerand having electric power output of electrons emitted by said filamentand received by said electron collecting electrode to effect subment tosaid electric current output means for enermeans and control means, saidregulator being re- 10 gizatiou thereby, said filament providing aquantity sponsive to variations in the magnitude or" electric ofelectrons which varies in magnitude in the same voltage supplied to saidcontrol means to provide a direction as variations in the magnitude ofcurrent variation in the same direction in the magnitude of fromsaidoutput means; electric current delivered by said output means; (0) anauxiliary source of electric power having out- (b) electricallyconductive means connecting said filaput means shunted by a zener diode,said auxiliary ment to said electric power output means for encrso rceproviding a delay between the time of energization thereby, saidfilament providing a quantity grzation thereof and a predetermined timeat which of electrons which varies in magnitude in the same full outputvoltage is reached, said output voltage direction as variations in themagnitude of current increasing gradually to full output voltage asdeterfrom said output means; by said zener diode and thereafter beinges- (c) an auxiliary source or" electric power having outsentiallyconstant; and

put means, said auxiliary source providing a delay (d) conductive meansconnecting said electron colbetween the time of energization thereof anda prelecting electrode in series circuit with a constant voltdeterminedtime at which full output voltage is source, in series circuit with theoutput means reached, said output voltage increasing gradually to ofsaid auxiliary source and to said control means, full output voltage andthereafter being essentially said auxiliary source being arranged toprovide an constant; and electric voltage to said control means whicheffects Conductive 1621115 n ting said electron collecta gradualincrease in the power supplied to said filaing electrode in seriescircuit with a constant vol age ment until said pr ned time elapses andtheresource, in series circuit with the output means of 39 after providea constant magnitude input voltage to Said auXiliaiy $011K: and to Saidmeans, i said control means, said electron collecting electrodeauxiliary source being arranged to provide an elecbeing arranged toprovide variations in the magnitric voltage to said control means whicheffects a tude of power to said control means which are in the gradualincrease in the current supplied to said filaopposite direction tovariations in the quantity of ment until said predetermined time elapsesand thereelectrons emitted by said filament and received by afterProvide a Constant magniilldfi input Voltagfi t0 said electroncollecting electrode to effect substansaid control means, said electroncollecting electrode tially o t t emission current from aid fil t beingarranged to provide variations in the magniafter said predetermined timehas elapsed. tude of voltage to said control means which are in theopposite direction to variations in the quantity 40 References Cited bythe Examiner UNITED STATES PATENTS stantially constant emission currentfrom said filagizg g 1 a ment after said p ed t rmm fi has elapse2,942,123 6/60 Schuh 307 88,5

3. An emission current regulated power supply for the filament of a massspectrometer having a thermionic electron emission filament and anelectron collecting electrode, said power supply comprising:

RALPH G. NELSGN, Prirrzary Examiner.

1. AN EMISSION CURRENT REGULATED POWER SUPPLY FOR THE FILAMENT OF A MASSSPECTROMETER HAVING A THERMIONIC ELECTRON EMISSIVE FILAMENT AND ANELECTRON COLLECTING ELECTRODE, SAID POWER SUPPLY COMPRISING: (A) ANELECTRIC POWER REGULATOR INCLUDING A SOURCE OF ELECTRIC POWER AND HAVINGELECTRIC POWER OUTPUT MEANS AND CONTROL MEANS, REGULATOR BEINGRESPONSIVE TO VARIATIONS IN THE MAGNITUDE OF ELECTRIC POWER SUPPLIED TOSAID CONTROL MEANS TO PROVIDE A VARIATION IN THE SAME DIRECTION IN THEMAGNITUDE OF ELECTRIC POWER DELIVERED BY SAID OUTPUT MEANS; (B)ELECTRICALLY CONDUCTIVE MEANS CONNECTING SAID FILAMENT TO SAID ELECTRICPOWER MEANS FOR ENERGIZATION THEREBY, SAID FILAMENT PROVIDING A QUANTITYOF ELECTRONS WHICH VARIES IN MAGNUTUDE IN THE SAME DIRECTION ASVARIATIONS IN THE MAGNIDUDE OF POWER FROM SAID OUTPUT MEANS; (C) ANAUXIALLY SOURCE OF ELECTRIC POWER HAVING OUTPUT MEANS, SAID AUXILIARYSOURCE PROVIDING A DELAY BETWEEN THE TIME OF ENERGIZATION THEREOF AND APREDETERMINED TIME AT WHICH FULL OUTPUT POWER CAPABILITY IS REACHED,SAID OUTPUT POWER CAPABILITY INCREASING GRADUALLY TO FULL OUTPUT POWERCAPABILITY AND THEREAFTER BEING ESSENTIALLY CONSTANT; AND (D) CONDUCTIVEMEANS CONNECTING SAID ELECTRON COLLECTING ELECTRODE AND THE OUTPUT MEANSOF SAID AUXILIARY SOURCE TO SAID CONTROL MEANS, SAID AUXILIARY SOURCEBEING ARRANGED TO PROVIDE AN ELECTRIC POWER INPUT TO SAID CONTROL MEANSWHICH EFFECTS A GRADUAL INCREASE IN THE POWER SUPPLIED TO SAID FILAMENTUNTIL SAID PREDETERMINED TIME ELAPSES AND THEREAFTER PROVIDE A CONSTANTMAGNITUDE POWER INPUT TO SAID CONTROL MEANS, SAID ELECTRON COLLECTINGELECTRODE BEING ARRANGED TO PROVIDE VARIATIONS IN THE MAGNITUDE OF POWERTO SAID CONTROL MEANS WHICH ARE IN THE OPPOSITE DIRECTION TO VARIATIONSIN THE QUANTITY OF ELECTRONS EMITTED BY SAID FILAMENT AND RECEIVED BYSAID ELECTRON COLLECTING ELECTRODE TO EFFECT SUBSTANTIALLY CONSTANTEMISSION CURRENT FROM SAID FILAMENT AFTER SAID PREDETERMINED TIME HASELAPSED.